Support for format args capture

build.rs

@@ -58,6 +58,10 @@ fn main() {
     if rustc < 51 {
         println!("cargo:rustc-cfg=anyhow_no_ptr_addr_of");
     }
+
+    if rustc < 52 {
+        println!("cargo:rustc-cfg=anyhow_no_fmt_arguments_as_str");
+    }
 }
 
 fn compile_probe() -> Option<ExitStatus> {

src/lib.rs

@@ -604,9 +604,13 @@ pub trait Context<T, E>: context::private::Sealed {
 // Not public API. Referenced by macro-generated code.
 #[doc(hidden)]
 pub mod private {
+    use crate::Error;
+    use alloc::fmt;
+    use core::fmt::{Arguments, Debug, Display};
+
     pub use alloc::format;
     pub use core::result::Result::Err;
-    pub use core::{concat, stringify};
+    pub use core::{concat, format_args, stringify};
 
     #[doc(hidden)]
     pub mod kind {
@@ -615,4 +619,24 @@ pub mod private {
         #[cfg(feature = "std")]
         pub use crate::kind::BoxedKind;
     }
+
+    #[doc(hidden)]
+    #[cold]
+    pub fn format_err<M>(message: M, args: Arguments) -> Error
+    where
+        M: Display + Debug + Send + Sync + 'static,
+    {
+        #[cfg(anyhow_no_fmt_arguments_as_str)]
+        let has_interpolated_format_args = false;
+        #[cfg(not(anyhow_no_fmt_arguments_as_str))]
+        let has_interpolated_format_args = args.as_str().is_none();
+
+        if has_interpolated_format_args {
+            // anyhow!("interpolate {var}"), can downcast to String
+            Error::msg(fmt::format(args))
+        } else {
+            // anyhow!("literal"), can downcast to &'static str
+            Error::msg(message)
+        }
+    }
 }

src/macros.rs

@@ -168,9 +168,7 @@ macro_rules! ensure {
 #[macro_export]
 macro_rules! anyhow {
     ($msg:literal $(,)?) => {
-        // Handle $:literal as a special case to make cargo-expanded code more
-        // concise in the common case.
-        $crate::Error::msg($msg)
+        $crate::private::format_err($msg, $crate::private::format_args!($msg))
     };
     ($err:expr $(,)?) => ({
         use $crate::private::kind::*;